This application is based on and claims the priority from the prior Japanese Patent Application No. 2000-082590, filed Mar. 23, 2000; Application No. 2001-065247, filed Mar. 8, 2001; and Application No. 2001-085820, filed Mar. 23, 2001, the entire contents of which are incorporated herein by reference.
The present invention relates to a method and apparatus for measuring a characteristic of a specimen of a magnetic head and its application to high frequency response measurement with the scanning probe microscopes.
In the process of manufacturing magnetic recording heads used in, for example, hard disk drives, a dedicated magnetic recording head measuring apparatus using a magnetic force microscope (MFM) has been used to measure the distribution of the magnetic field of a head and its magnetic force. The MFM, which is a kind of scanning probe microscope, normally brings a probe provided on a cantilever close to a magnetic recording head as a specimen to be measured, and senses the magnetic force interaction (force or force gradient) caused by the magnetic field generated from the head in a noncontacting manner.
The magnetic recording head is, for example, an inductive thin-film head and has a magnetic gap that generates a recording magnetic field according to a signal current applied to a coil. The magnetic recording head measuring apparatus applies a high-frequency signal current to (the coil of) the head as a specimen, and measures the distribution of the magnetic field generated from the magnetic gap. One known measuring method is to sense the phase or displacement of the vibrating cantilever (i.e., the magnetic force interaction caused by the magnetic field of the head) and measure the force gradient or force between the probe and the specimen on the basis of the result of the sensing. The phase shift of the cantilever approximates the force gradient and the displacement of the cantilever approximates the force.
As described above, the magnetic recording head measuring apparatus using an MFM measures the force or force gradient between the head and the probe when causing the head to generate a magnetic field, thereby measuring the magnetic force of the head. The frequency of the signal current to cause the head to generate a magnetic field in measurement is high (for example, several tens of MHz), which is much higher than the resonance frequency of the cantilever that determines the response speed of the measurement. For this reason, it is difficult to measure the magnetic force of the head with a high sensitivity and a high resolution by extracting only the high-frequency response in measuring the magnetic force of the head on the basis of the resonance frequency component of the cantilever.
Another known measuring method is to apply a high-frequency sinusoidal wave to the head and measure the DC components of the phase shift (corresponding to force gradient) of the cantilever caused by the magnetic field generated from the head. Since the measured DC components include the components other than the high-frequency components, it is difficult to interpret the measured data. In addition, a known force measuring method of measuring the force from the displacement of the cantilever has the problems of relatively low resolution and variations in the distance between the probe and the specimen in places with the strong and weak magnetic fields.
In the meantime, H. Yokoyama and T. Inoue, Thin Solid Films 242 (1994) 33 disclose a technique of applying an amplitude-modulated voltage with a modulation frequency and a carrier frequency to a specimen and measuring an electrostatic force interaction of a high frequency component between the probe and the specimen. R. Proksch and P. Neilson, S. Austvold and J. J. Schmidt, Applied Physics Letters 74 (1999) 1308. (Digital Instruments et al.) also disclose a similar technique.
However, in these documents, an interaction between the probe and the specimen caused by the excitation field generated at the surface of the specimen is detected as a displacement (vibration amplitude) of the cantilever, so that the interaction is small when the excitation field is weak and the interaction is large when the excitation field is strong. As a result, it is difficult to obtain an image to which the distribution of the excitation field is precisely reflected.
Furthermore, in these documents, the displacement of the cantilever is determined by the intensity of the interaction, so that a back portion of the probe is particularly influenced by the interaction. As a result, it is difficult to obtain a high spatial resolution in the measurement.
In addition, a conventional measuring method is mainly for the spatial distribution of the magnetic field of a magnetic recording head and is unsuitable for measuring the frequency characteristic of a high-frequency magnetic field in a state where the magnetic recording head is actually incorporated in an HDD. The reason is the difference in signal applying method between a signal generator and a head amplifier for driving the magnetic recording head. The signal generator is normally operated at a constant voltage, whereas the actual head amplifier is operated at a constant current (although the voltage and the current might not be constant at high frequencies because of the frequency characteristic). Since the impedance of the circuit meter varies at high frequencies, the frequency characteristic in a state similar to a case where the magnetic recording head is incorporated in an HDD cannot be measured. In addition, since the actual head amplifier operates digitally, it is difficult to incorporate the head amplifier in a conventional magnetic recording head measuring apparatus.
Accordingly, it is an object of the present invention to provide a method and apparatus for measuring a characteristic of a specimen, which realize high-frequency response and measurement with a high sensitivity and a high resolution.
It is another object of the present invention to provide a method and apparatus capable of measuring a frequency characteristic of the magnetic field generated from a magnetic head in an environment almost similar to a state where the magnetic recording head is incorporated in an HDD.
According to one aspect of the present invention, there is provided an apparatus for measuring a characteristic of a specimen, comprising: a probe for scanning a surface of the specimen in a noncontacting state; vibrating means for vibrating the probe; excitation field generating means for generating an amplitude modulation signal which is amplitude-modulated with a modulation frequency and a carrier frequency and producing an excitation field at the surface of the specimen on the basis of the generated amplitude modulation signal; and measuring means for measuring a change in a vibration frequency of the probe according to the excitation field generated at the surface of the specimen.
According to another aspect of the present invention, there is provided an apparatus for measuring a characteristic of a specimen, comprising: a probe for scanning a surface of the specimen in a noncontacting state; vibrating means for vibrating the probe; excitation field generating means for generating an amplitude modulation signal which is amplitude-modulated with a modulation frequency and a carrier frequency and producing an excitation field at the surface of the specimen on the basis of the generated amplitude modulation signal; and measuring means for measuring a force gradient induced to the probe by the excitation field generated at the surface of the specimen.
According to still another aspect of the present invention, there is provided an apparatus for measuring a characteristic of a magnetic recording head, comprising: a probe for scanning a surface of the magnetic recording head in a noncontacting state; vibrating means for vibrating the probe; current generating means for generating an amplitude modulation current and applying the generated current to the magnetic recording head; and measuring means for measuring a force gradient induced to the probe by the magnetic field generated at the surface of the magnetic recording head according to the application of the amplitude modulation current.
According to still another aspect of the present invention, there is provided an apparatus for measuring a characteristic of a magnetic recording head, comprising: a probe for scanning a surface of the magnetic recording head in a noncontacting state; vibrating means for vibrating the probe; signal generating means for generating an amplitude modulation signal on the basis of a carrier frequency and varying a value of the amplitude modulation signal by changing a value of the carrier frequency; a head amplifier equivalent circuit for generating an amplitude modulation current to cause the magnetic recording head to operate on the basis of the amplitude modulation signal and applying the generated current to the magnetic recording head, the head amplifier equivalent circuit having an electrical characteristic equivalent to an actual head driving amplifier; phase shift measuring means for measuring a phase shift of a vibration of the probe according to the magnetic field generated by the magnetic recording head; and magnetic field frequency dependence measuring means for measuring a change in a value of the phase shift with respect to a change in the value of the amplitude modulation signal caused by the signal generating means as a magnetic field frequency dependence of the magnetic recording head.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.